Cystic fibrosis (CF) is a lethal, recessive, genetic disease affecting approximately 1 in 2500 live births among Caucasians. (Cohen-Cymberknoh, M. et al., Am. J. Respir. Crit. Care Med. 1463-1471, 2011; Boat et al., The Metabolic Basis of Inherited Disease, 6th ed., pp 2649-2680, McGraw Hill, NY (1989)). Approximately 1 in 25 persons are carriers of the disease. The major symptoms of cystic fibrosis include chronic pulmonary disease, pancreatic exocrine insufficiency, and elevated sweat electrolyte levels. The symptoms are consistent with cystic fibrosis being an exocrine disorder. (Hantash F: U.S. Patent Application No. 20060057593). The CF gene codes for a cAMP/PKA-dependent, ATP-requiring, membrane chloride ion channel, generally found in the apical membranes of many secreting epithelia and is known as CFTR (cystic fibrosis transmembrane conductance regulator). There are currently over 1900 known mutations affecting CFTR, many of which give rise to a disease phenotype. Around 75% of CF alleles contain the ΔF508 mutation in which a triplet codon has been lost, leading to a missing phenylalanine at position 508 in the protein. This altered protein fails to be trafficked to the correct location in the cell and is generally destroyed by the proteasome. The small amount that does reach the correct location functions poorly. (Cuthbert A W, British Journal of Pharmacology, 163(1), 173-183, 2011).
Mutations in the CFTR gene result in absence or dysfunction of the protein that regulates ion transport across the apical membrane at the surface of certain epithelia. Although CFTR functions mainly as a chloride channel, it has many other roles, including inhibition of sodium transport through the epithelial sodium channel, regulation of the outwardly rectifying chloride channel, ATP channels, intracellular vesicle transport, and inhibition of endogenous calcium-activated chloride channels. CFTR is also involved in bicarbonate-chloride exchange. A deficiency in bicarbonate secretion leads to poor solubility and aggregation of luminal mucins. Obstruction of intrapancreatic ducts with thickened secretions causes autolysis of pancreatic tissue with replacement of the body of the pancreas with fat, leading to pancreatic insufficiency with subsequent malnutrition. In the lungs, CFTR dysfunction leads to airway surface liquid (ASL) depletion and thickened and viscous mucus that adheres to airway surfaces. The result is decreased mucociliary clearance (MCC) and impaired host defenses. Dehydrated, thickened secretions lead to endobronchial infection with a limited spectrum of distinctive bacteria, mainly Staphylococcus aureus and Pseudomonas aeruginosa, and an exaggerated inflammatory response leading to development of bronchiectasis and progressive obstructive airways disease. Pulmonary insufficiency is responsible for most CF-related deaths. (Cohen-Cymberknoh, M et al., Am. J. Respir. Crit. Care Med. 1463-1471, 2011).
The prognosis for the treatment of CF has improved over the last 40 years. This was achieved by improving pancreatic enzyme supplements, drugs designed to treat pulmonary infection, reduce inflammation and enhance mucociliary clearance. Currently the therapeutic challenges are to correct the biochemical defect of CF and to identify effective treatments for chronic respiratory infection. (Frerichs C. et al., Expert Opin Pharmacother. 10(7), 1191-202, 2009).